Introduction

In coordination chemistry, Schiff base complexes are an essential class of compounds. They are displayed to be very effective in catalyzing the electrochemical reaction, resulting in a change in over-potentials and peak currents. Many reports show the application of these complexes as mediator in the electrochemical determination of pharmaceutical and biological compounds [1–5]. Cobalt Schiff base complexes were also applied as modifier of nanoclay compounds. Surface modification of clay minerals has earned consideration because of production of new substances and applications. Modified clays can be used in different areas such as adsorbents of organic pollutants in soil, water and air; rheological control agents; paints; cosmetics; refractory varnish; thixotropic fluids; and polymer nanocomposite [6, 7].

Cobalt Schiff base complexes have been studied as models for the cobalamine coenzymes (B12) [8] and categorized as an oxygen carrier [9]. They were generally used as a catalyst for the preparative oxygenation of phenols [10] and amines [11]. Formerly studies have investigated the catalytic activity of the Co(III) salen with the active species in which cobalt has the oxidation state of 3 [12]. Comprehensive electrochemical studies of [CoL(PR3)]+ (where L = tetradentate N2O2 Schiff bases) complexes were carried out previously [13–22]. Our studies on cobalt(III) complexes coordinated by tetradentate Schiff base ligands and phosphine with formula of [CoL(PR3)(H2O)]+ (where L = tetradentate N2O2 Schiff bases) show a hexa-coordinated geometry where PR3 and H2O are in trans position relative to each other [7]. On the other hand, in some cases, solvent competes with water and the complexes with general formula of [CoL(PR3)(EtOH)]+ are formed [6].

To extend our investigations on the structure of unsymmetrical Schiff base complexes [23–25], some tetradentate unsymmetrical Schiff base ligands were prepared from 2-hydroxyacetophenone, 1,2 phenylenediamine and salicylaldehyde derivatives, and then, the cobalt(III) complexes of these ligands with formula of [CoL(PR3)(CH3OH)]ClO4 (where L = MeSalophen, 5-NO2MeSalophen, 5-BrMeSalophen, 5-MeOMeSalophen, 4-MeOMeSalophen, 3-MeOMeSalophen and R = Ph, Bu) were synthesized in dried methanol (Fig. 1). The characterization of the synthesized complexes were performed by FTIR, 1H NMR, 13C NMR and UV–Vis spectroscopies. The coordination geometry of [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 and [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4 was determined by X-ray crystallography. We have also investigated the electronic spectra and the electrochemical behavior of Co(III) MeSalophen complexes with the aim of evaluating the effects of exchange of the axial and equatorial ligands.

Fig. 1
figure 1

Structure of [Co(X-MeSalophen)(PR3)(CH3OH)]ClO4 complexes, X = H, 5-NO2, 5-Br, 5-MeO, 4-MeO and 3-MeO, R = Bu and Ph3

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Experimental

Materials

All chemicals and solvents were purchased from commercial sources without any further purification. The asymmetrical tetradentate Schiff bases were prepared by a previously reported procedure and characterized by various spectroscopic techniques [23–25].

Characterizations

Fourier transform infrared (FTIR) spectra were recorded as KBr disks on a FTIR JASCO-680 spectrophotometer in the 400–4000 cm−1. UV–Vis spectra were recorded on a JASCO V-570 spectrophotometer in the 200–800 nm. The 1H and 13C NMR spectra were recorded in DMSO-d 6 on Bruker-500 MHz. Cyclic voltammograms were performed by using autolab modelar electrochemical system (ECO Chemie, Utrecht, the Netherlands) equipped with a PSTA 20 module and driven by GPES (ECO Chemie) in conjunction with a three-electrode system and a PC for data processing. An Ag/AgCl (saturated KCl)/3 M KCl reference electrode, a Pt wire as counterelectrode and a glassy carbon electrode as working electrode (Metrohm glassy carbon, 0.0314 cm2) were used for the electrochemical studies. Voltammetric measurements were taken at room temperature in DMF solution with 0.1 M tetrabutylammonium perchlorate as the supporting electrolyte.

Synthesis of the metal Schiff base complexes

Cobalt complexes were synthesized according to the following general procedure: cobalt(II) acetate tetrahydrate (0.249 g, 1.0 mmol) and desired phosphine (1.0 mmol) were added to a methanolic solution (50 mL) of ligand (1.0 mmol), and it was refluxed for 1 h. The synthesized Co(II) complex was oxidized by air blowing for 2 h and filtered. To the filtrate, an appropriate amount of sodium perchlorate (0.140 g, 1.0 mmol) was added and suitable crystals were formed after 5 days.

[Co(MeSalophen)(PPh3)(CH3OH)]ClO4, yield (75 %). FTIR (KBr cm−1) ν max 1600 (C=N), 1440, 1518 (C=C), 1200 (C–O), 1090 (ClO4). 1H NMR (DMSO-d 6 , δ, ppm): 2.9 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 6.6–7.9 (m, 27H, Ar–H), 8.6 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 39.7 (CH3), 48.6 (CH3OH), 115.4, 115.6, 117.0, 118.0, 121.0, 122.0, 123.9, 123.9, 124.0, 126.0, 127.0, 128.0, 131.5, 131.7, 133.4, 133.5, 134.0, 135.0, 136.0, 144.0, 145.0 and 160.0 (C–Ar), 164.0 and 176.0 (HC=N). UV–Vis, λ max (nm) (methanol): 250, 380, 500, 684.

[Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4, yield (85 %). FTIR (KBr cm−1) ν max 1600 (C=N), 1440, 1510 (C=C), 1200 (C–O), 1090 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 2.9 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 6.6–7.9 (m, 26H, Ar–H), 8.7 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 39.5 (CH3), 48.0 (CH3OH), 105.0, 115.0, 117.0, 120.0, 121.0, 123.6, 124.0, 124.0, 125.0, 127.0, 128.3, 128.4, 131.6, 131.8, 133.4, 133.5, 134.0, 136.0, 138.0, 144.0, 145.0 and 159.0 (C–Ar), 163.0 and 177.0 (HC=N). UV–Vis, λ max (nm) (methanol): 254, 380, 502, 683.

[Co(5-NO2MeSalophen)(PPh3)(CH3OH)]ClO4, yield (83 %). FTIR (KBr cm−1) ν max 1600 (C=N), 1438, 1540 (C=C), 1390, 1310 (NO2), 1200 (C–O), 1090 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 2.9 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 6.6–8.5 (m, 26H, Ar–H), 8.9 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 39.5 (CH3), 48.0 (CH3OH), 115.0, 117.0, 121.0, 123.2, 123.7, 124.0, 127.0, 128.5, 128.6, 128.7, 129.0, 131.4, 131.9, 132.0, 133.4, 133.5, 135.0, 136.0, 144.0, 145.0, 161.0 and 163.0 (C–Ar), 169.0 and 177.0 (HC=N). UV–Vis, λ max (nm) (methanol): 245, 354, 457, 656.

[Co(MeSalophen)(PBu3)(CH3OH)]ClO4, yield (80 %). FTIR (KBr cm−1) ν max 2950 (Bu(C-H)), 1600 (C=N), 1475, 1518 (C=C), 1200 (C–O), 1090 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 0.7–0.9 (t, 9H, CH3), 0.9–1.6 (m, 18H, CH2), 3.0 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 6.7–8.2 (m, 12H, Ar–H), 9.0 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 13.1 (CH3), 13.5, 23.4 and 23.4 (CH2), 23.9 (CH3), 39.6 (CH3OH), 105.0, 115.0, 117.0, 121.0, 122.0, 123.0, 124.0, 125.0, 127.0, 128.0, 131.0, 135.0, 136.0, 138.0, 144.0, 145.0, 159.0 and 164.7 (C–Ar), 164.9 and 177.0 (HC=N). UV–Vis, λ max (nm) (methanol): 257, 385, 484, 582.

[Co(5-BrMeSalophen)(PBu3)(CH3OH)]ClO4, yield (86 %). FTIR (KBr cm−1) ν max 2960 (Bu(C-H)), 1600 (C=N), 1475, 1520 (C=C), 1200 (C–O), 1090 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 0.65–0.7 (t, 9H, CH3), 0.8–1.6 (m, 18H, CH2), 3.0 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 6.7–8.2 (m, 11H, Ar–H), 9.0 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 13.1 (CH3), 13.6, 23.5 and 23.8 (CH2), 23.9 (CH3), 40.0 (CH3OH), 105.0, 115.0, 117.0, 121.0, 122.0, 123.0, 124.0, 125.0, 127.0, 128.0, 131.0, 135.0, 136.0, 138.0, 144.0, 145.0, 159.0 and 164.7 (C–Ar), 164.9 and 177.0 (HC=N). UV–Vis, λ max (nm) (methanol): 256, 380, 484, 550.

[Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4, yield (70 %). FTIR (KBr cm−1) ν max 2960 (Bu(C-H)), 1600 (C=N), 1460, 1540 (C=C), 1320, 1430 (NO2), 1200 (C–O), 1100 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 0.6–0.7 (t, 9H, CH3), 1.1–1.3 (m, 18H, CH2), 3.0 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 6.7–8.7 (m, 11H, Ar–H), 9.3 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 13.1 (CH3), 23.4, 23.8 and 23.9 (CH2), 24.0 (CH3), 39.5 (CH3OH), 115.0, 117.0, 119.0, 122.4, 122.9, 123.0, 125.0, 128.3, 128.4, 129.0, 132.0, 133.0, 135.0, 136.0, 141.0, 145.0, 161.0 and 164.0 (C–Ar), 170.7 and 178.4 (HC=N). UV–Vis, λ max (nm) (methanol): 254, 356, 458, 532.

[Co(5-MeOMeSalophen)(PPh3)(CH3OH)]ClO4, yield (90 %). FTIR (KBr cm−1) ν max 1598 (C=N), 1440, 1518 (C=C), 1200 (C–O), 1090 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 2.5 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 3.7 (s, 3H, OMe), 6.1–7.9 (m, 26H, Ar–H), 8.6 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 24.0 (CH3), 39.6 (CH3OH), 55.4 (OCH3), 113.0, 115.0, 117.0, 118.0, 121.0, 123.8, 123.9, 124.1, 124.5, 126.0, 127.0, 127.5, 128.2, 128.3, 131.0, 133.4, 133.5, 144.0, 145.0, 149.0, 159.0 and 163.0 (C–Ar), 173.0 and 176.0 (HC=N). UV–Vis, λ max (nm) (methanol): 254, 388, 524, 710.

[Co(4-MeOMeSalophen)(PPh3)(CH3OH)]ClO4, yield (85 %). FTIR (KBr cm−1) ν max 1608 (C=N), 1440, 1510 (C=C), 1310 (C–O), 1090 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 2.9 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 3.7 (s, 3H, OMe), 6.2–7.8 (m, 26H, Ar–H), 8.45 (s, HC=N). UV–Vis, λ max (nm) (methanol): 256, 320, 495, 707.

[Co(3-MeOMeSalophen)(PPh3)(CH3OH)]ClO4, yield (90 %). FTIR (KBr cm−1) ν max 1598 (C=N), 1438, 1540 (C=C), 1200 (C–O), 1090 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 2.9 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 3.9 (s, 3H, OMe), 6.4–8.0 (m, 26H, Ar–H), 8.5 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 39.0 (CH3), 48.6 (CH3OH), 55.9 (OCH3), 114.0, 115.0, 115.3, 117.0, 118.0, 121.0, 123.5, 123.7, 123.9, 124.0, 125.0, 126.0, 127.0, 128.0, 131.0, 133.0, 134.0, 144.0, 145.0, 154.1, 154.5 and 159.0 (C–Ar), 164.0 and 176.0 (HC=N). UV–Vis, λ max (nm) (methanol): 252, 390, 515, 718.

[Co(5-MeOMeSalophen)(PBu3)(CH3OH)]ClO4, yield (90 %). FTIR (KBr cm−1) ν max 2940 (Bu(C-H)), 1598 (C=N), 1460, 1520 (C=C), 1220 (C–O), 1080 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 0.4–0.4 (t, 9H, CH3), 0.6–1.4 (m, 18H, CH2), 2.7 (s, 3H, CH3), 3.2 (s, 3H, CH3OH), 3.4 (s, 3H, OMe), 6.6–8.2 (m, 11H, Ar–H), 8.7 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 13.1 (CH3), 13.5, 21.2 and 23.5 (CH2), 23.6 (CH3), 40.2 (CH3OH), 55.5 (OMe), 113.0, 115.0, 117.0, 118.0, 122.0, 123.1, 123.4, 124.0, 127.0, 127.2, 129.0, 131.0, 135.0, 144.0, 145.0, 149.0, 159.0 and 161.0 (C–Ar), 165.1 and 177.3 (HC=N). UV–Vis, λ max (nm) (Methanol): 254, 385, 509, 650.

[Co(4-MeOMeSalophen)(PBu3)(CH3OH)]ClO4, yield (80 %). FTIR (KBr cm−1) ν max 2950 (Bu(C–H)), 1604 (C=N), 1458, 1518 (C=C), 1218 (C–O), 1090 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 0.6–O.7 (t, 9H, CH3), 0.8–1.6 (m, 18H, CH2), 2.9 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 3.8 (s, 3H, OMe), 6.3–8.2 (m, 11H, Ar–H), 8.8 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 13.1 (CH3), 23.4, 23.6 and 23.8 (CH2), 39.6 (CH3), 40.0 (CH3OH), 55.5 (OMe), 103.0, 107.0, 114.0, 115.0, 117.0, 122.0, 123.0, 124.0, 126.0, 127.0, 131.0, 134.0, 136.0, 144.0, 146.0, 158.0, 165.0, and 166.0 (C–Ar), 168.0 and 177.1 (HC=N). UV–Vis, λ max (nm) (Methanol): 256, 324, 380, 627.

[Co(3-MeOMeSalophen)(PBu3)(CH3OH)]ClO4, yield (85 %). FTIR (KBr cm−1) ν max 2950 (Bu(C–H)), 1600 (C=N), 1438, 1518 (C=C), 1240 (C–O), 1080 (ClO4). 1H NMR (DMSO-d6, δ, ppm): 0.6–0.7 (t, 9H, CH3), 1.1–1.2 (m, 18H, CH2), 3.0 (s, 3H, CH3), 3.4 (s, 3H, CH3OH), 3.8 (s, OMe), 6.6–8.2 (m, 11H, Ar–H), 8.9 (s, HC=N). 13C NMR (DMSO-d6, δ, ppm): 13.1 (CH3), 23.4, 23.6 and 23.8 (CH2), 23.9 (CH3), 39.5 (CH3OH), 55.8 (OMe), 114.0, 115.2, 115.5, 117.0, 119.0, 122.0, 123.0, 124.0, 126.0, 127.0, 127.9, 131.0, 135.0, 144.0, 145.0, 152.0, 156.0 and 160.0 (C–Ar), 164.0 and 177.4 (HC=N). UV–Vis, λ max (nm) (methanol): 250, 324, 509, 695.

Crystal structure determination of [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 and [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4 complexes

The intensity data were collected on a Nonius Kappa CCD diffractometer, using graphite-monochromated Mo-K α radiation. Data were corrected for Lorentz and polarization effects, but not for absorption [26, 27].

The structure was solved by direct methods (SHELXS) [28] and refined by full-matrix least-squares techniques against Fo2 (SHELXL-97) [28]. Both crystal structures contain large voids, filled with disordered solvent molecules. The size of the voids is 58 and 57 Å3/unit cell, respectively. Their contribution to the structure factors was secured by back-Fourier transformation using the SQUEEZE routine of the program PLATON [29] resulting in 27 and 14 electrons/unit cell, respectively.

The hydrogen atom of the hydroxy group of the methanol molecule of [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 was located by difference Fourier synthesis and refined isotropically. All other hydrogen atoms were included at calculated positions with fixed thermal parameters. All non-disordered, non-hydrogen atoms were refined anisotropically [3]. XP (SIEMENS Analytical X-ray Instruments, Inc.) was used for structure representations.

Crystal data for [Co(5-NO2MeSalophen)(PBu3)CH3OH]ClO4: [C34H45CoN3O5P]+, ClO4 − [*], Mr = 765.08 g mol−1 [*], brown prism, size 0.038 × 0.036 × 0.034 mm3, monoclinic, space group P 21/n, a = 12.5144 (2), b = 23.6651 (4), c = 12.6002 (2) Å, β = 91.228 (1)°, V = 3730.75 (11) Å3, T = −140 °C, Z = 4, ρ calcd. = 1.362 g cm−3 [*], µ (Mo-K α) = 6.29 cm−1 [*], multi-scan, transmin: 0.7066, transmax: 0.7456, F(000) = 1604 [*], 8214 reflections in h(−16/16), k(−30/30), l(−16/16), measured in the range 2.36° ≤ θ ≤ 27.10°, completeness θ max = 99.8 %, 8214 independent reflections, R int = 0.0409, 7221 reflections with F o  > 4σ(F o ), 448 parameters, 0 restraints, R 1obs = 0.0557, wR 2obs = 0.1514, R 1all = 0.0635, wR 2all = 0.1570, GOOF = 1.025, largest difference peak and hole: 1.181/−0.688 e Å−3.

Crystal data for [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4:[C40H34.3BrCoN2O3.15P]+ ClO4 − [*], Mr = 862.65 g mol−1[*], brown prism, size 0.044 × 0.034 × 0.034 mm3, triclinic, space group P \( \bar{1} \), a = 15.1709 (3), b = 22.2123 (4), c = 23.4763 (3) Å, α = 75.670 (1), β = 87.769 (1), γ = 75.864 (1)°, V = 7430.9 (2) Å3, T = −140 °C, Z = 8, ρ calcd. = 1.542 g cm−3 [*], µ (Mo-K α) = 17.07 cm−1 [*], multi-scan, transmin: 0.6888, transmax: 0.7455, F(000) = 3516 [*], 52,033 reflections in h(−19/19), k(−28/28), l(−30/30), measured in the range 1.82° ≤ θ ≤ 27.10°, completeness θ max = 94.9 %, 31,147 independent reflections, R int = 0.0358, 24,071 reflections with F o  > 4σ(F o ), 1889 parameters, 0 restraints, R 1obs = 0.0791, wR 2obs = 0.1849, R 1all = 0.1050, wR 2all = 0.2000, GOOF = 1.058, largest difference peak and hole: 3.841/−1.184 e Å−3.

[*] derived parameters do not contain the contribution of the disordered solvent.

Results and discussion

FTIR characterization

In the FTIR spectra of the Schiff base ligands [23–25] and cobalt(III) complexes, various bands were observed in the 400–4000 cm−1 region. A FTIR spectra of [Co(MeSalophen)(PBu3)(CH3OH)]ClO4 complex is shown in Fig. 2. The azomethine vibration of the Schiff base ligands appeared at 1610–1618 cm−1 [23–25]. Bond formation between the metal and the imine nitrogen leads to a shift of the C=N bond stretching to lower frequencies relative to the free Schiff base and appeared in 1598–1608 cm−1 for cobalt(III) complexes [30]. The C–H stretching of PBu3 coordinated ligands appeared in the 2940–2960 cm−1 region. The stretching vibration of ClO4 − counterion appeared at 1080–1100 cm−1.

Fig. 2
figure 2

FTIR of [Co(MeSalophen)(PBu3)(CH3OH)]ClO4 complexes

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Electronic spectra

The UV–Vis spectral data (200–800 nm) of synthesized complexes are listed in “Experimental” section. The electronic spectra of cobalt(III) complexes showed four absorption regions. It is shown in Fig. 3. All transitions related to aromatic ring involved π → π* were lower than 360 nm. In the Schiff base ligands, the band at 328–313 nm involves π → π* transition related to azomethine group [23–25]. This band was shifted to higher wavelengths and appeared at about 370–420 nm in mixed with charge transfer transition in cobalt complexes. The cobalt(III) complexes showed an absorption band related to charge transfer transition (MLCT) at about 380–524 nm region [31–37]. In addition, a d–d transition was observed at about 532–718 nm for the synthesized complexes [15, 16].

Fig. 3
figure 3

Electronic spectra of [Co(X-MeSalophen)(PBu3)(CH3OH)]ClO4 complexes. a X = 5-Br and (b, dash line) X = 5-NO2

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1H NMR and 13C NMR spectra

The 1H NMR data of the cobalt(III) complexes are presented in “Experimental” section. The aromatic hydrogens were observed in the range of 6.1–8.72 ppm. The imine hydrogen was appeared at 8.45–9.26 ppm. The hydrogens of the PBu3 in the [Co(MeSalophen)(PBu3)(CH3OH)]ClO4, [Co(5-BrMeSalophen)(PBu3)(CH3OH)]ClO4 and [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 appeared in the range of 0.65–0.88 ppm, while in the complexes containing methoxy group like [Co(5-MeOMeSalophen)(PBu3)(CH3OH)]ClO4, [Co(4-MeOMeSalophen)(PBu3)(CH3OH)]ClO4, [Co(3-MeOMeSalophen)(PBu3)(CH3OH)]ClO4 appeared in the range of 0.40–0.68 ppm. The 13C NMR techniques confirmed the structure of synthesized complexes as well. The carbons of PBu3 were observed in the range of 13.1–23.8 ppm. The aromatic carbons appeared in the range of 103–177.33 ppm, and the iminic carbon was seen in the range of 163–178.44 ppm.

Description of the molecular structure of the [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 and [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4

The structure of the aforementioned complexes was identified by X-ray diffraction and crystallized in the monoclinic, space group P 21 /n, and triclinic, space group P \( \bar{1} \), respectively. There is a list of X-ray diffraction data and selected bond lengths and angles of the complexes in Tables 1 and 2 for the mentioned complexes. The asymmetric unit of [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 and [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4 complexes is shown in Figs. 4 and 5, respectively.

Table 1 Crystallographic and structure refinements data for [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 (A), [Co(5-BrMeSalophen)(PPh3)CH3OH]ClO4 (B)
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Table 2 Selected bond distances (Å) and angle (°) for [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 (A), [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4 (B)
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Fig. 4
figure 4

Labeled diagram of [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4

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Fig. 5
figure 5

Labeled diagram of [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4

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In both complexes, each cobalt atom is coordinated in a distorted octahedral geometry and the Schiff base ligand with N2O2 donor atoms coordinated in the equatorial plane. The axial positions were occupied via phosphine and the methanol solvent. In contrast to our previously reported structures [6, 7] in this work, all of the complexes show the coordinated methanol solvent completely that also confirmed by NMR spectroscopy.

In the [Co(5-NO2MeSalophen)(PBu3)(CH3OH)]ClO4 complex, angles of P-Co–N/O were distributed from 95.95 (7)° to 89.47 (6)°, whereas O5–Co–N/O angles were distributed from 88.57 (9)° to 85.98 (8)°. The Co–N1, Co–N2, Co–O1 and Co–O2 distances also appeared in the range of 1.908 (2), 1.898 (2), 1.859 (19) and 1.894 (19) Å, respectively. There is a notable similarity with those in N2O2–salen cobalt complexes [36–38]. The Co–P bond distance of apical position is 2.2206 (7) for Co–P1. The Co–O1 and Co–O2 distances [1.859 (19), 1.894 (19) Å] are smaller than the Co–O5 distance 2.127 (2) Å, because of higher trans influence of the P1 atom with respect to the N1 and N2 atoms.

In the [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4 complex, angles of P–Co–N/O and O3–Co–N/O were distributed from 95.95 (7)° to 89.47 (6)° and 88.92 (16)° to 84.52 (15)°, respectively. The distances of Co–N1, Co–N2, Co–O1 and Co–O2 are 1.910 (5), 1.900 (4), 1.857 (4) and 1.902 (4) Å, respectively.

A significant deviation of all angles around the cobalt center from 90° indicates a regular distortion. The ligand–cobalt–ligand bond angles in the equatorial plane consist of two angles that are larger than 90° and two that are smaller than it (Tables 1 and 2) which are exactly similar to those in N2O2–salen cobalt complexes [39]. The summations of these angles are almost 360°, and it is a good proof that the cobalt atom is in a square planer environment of N2O2 atoms.

Electrochemical investigation

In this research, the impact of triphenyl and tributyl phosphine in both situations of axial ligand and equatorial Schiff base ligand on the electrochemical behavior of Co(III) Schiff base complexes was examined.

A typical cyclic voltammogram of [Co (MeSalophen)(PBu3)S]+ (where S = solvent) complex in the potential range from +0.2 to −1.5 +V (vs. Ag/AgCl) in DMF solution is shown in Fig. 6a. The first reduction peak was observed at about −0.565 V owing to the process.

$$ \left[ {{\text{Co}}^{\text{III}} \left( {\text{MeSalophen}} \right)\left( {{\text{PBu}}_{3} } \right){\text{S}}} \right]^{ + } + {\text{e}}^{ - } \to \, \left[ {{\text{Co}}^{\text{II}} \left( {\text{MeSalophen}} \right)\left( {{\text{PBu}}_{3} } \right)} \right] + {\text{S}} $$

The electron is added to the antibonding dz 2 orbital of the Co(III) complex, and in weak coordinating solvent, the product losses solvent. Upon reversal of the scan direction, the oxidation of the Co(II) complex to Co(III) was occurred at higher potentials (about 100 mV); then in a quick consecutive reaction, the [CoIII (MeSalophen)(PBu3)S]+ was formed again [40].

Fig. 6
figure 6

Cyclic voltammogram of a [Co(MeSalophen)(PBu3)(CH3OH)]ClO4 and (b, dash line) in presents of 0.001 M, PBu3 in DMF at room temperature. Scan rate: 100 mv/s

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The second quasi-reversible process at ca −1.465 V was noticed with about unit ratio of anodic to cathodic peak currents (i pa/i pc), corresponding to the simple one-electron process.

$$ \left[ {{\text{Co}}^{\text{II}} \left( {\text{MeSalophen}} \right)\left( {{\text{PBu}}_{ 3} } \right)} \right] + {\text{e}}^{ - } \to \left[ {{\text{Co}}^{\text{I}} \left( {\text{MeSalophen}} \right)\left( {{\text{PBu}}_{3} } \right)} \right]^{ - } $$

Multiple scans were formed in nearly superposable cyclic voltammograms, thereby showing the marked stability of the three oxidation states of cobalt involved in the electrochemical study. The reduction potentials for the different complexes are listed in Table 3.

Table 3 Reduction potential (in v) for [Co(X-MeSalophen)(PR3)(CH3OH)]ClO4, in DMF
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Figure 6b indicates the effect of PBu3 concentration on the electrochemical character of [CoIII (MeSalophen)(PBu3)S]+. Upon addition of tributyl phosphine (0.001 M), both of the oxidation peak potentials were occurred at lower potential, while there was a shift to negative direction for the reduction peak potentials (hexa-coordinated complex is formed). These changes prove that PBu3 in high concentration can be coordinated to Co(II) on the electrode area. They are also owing to an increase in antibonding character of the dz 2 orbital due to PR3 addition.

A cyclic voltammogram of a [CoIII(MeSalophen)(PPh3)S]+ complex is shown in Fig. 7a. Figure 7b indicates the effect of PPh3 concentration on the electrochemical character of [CoIII (MeSalophen)(PPh3)S]+. The results show that only the reduction potential of Co(III)–Co(II) is to be influenced by the concentration of PPh3. These changes prove that PBu3 in high concentration can be coordinated to both of Co(III) and Co(II) on the electrode area, while the PPh3 can only coordinated to Co(III).

Fig. 7
figure 7

Cyclic voltammogram of a [Co(MeSalophen)(PPh3)(CH3OH)]ClO4 and (b, dash line) in presents of 0.001 M, PPh3 in DMF at room temperature. Scan rate: 100 mv/s

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The nature of the axial ligand PR3 and the σ-donor strength of the phosphine axial ligand strongly influence the observed cathodic peak potentials E pc for the reduction process Co(III) + e− → Co(II) [40, 41]. There was also a clear reflection of the σ-donor strength of the various axial ligands in the spectra of the complexes. The energy of the transition between 532 and 718 nm descends in the order PBu3 > PPh3. There was a notable resemblance to this trend for the reduction of Co(III)–Co(II) and oxidation of Co(II)–Co(III). The complexes containing PBu3 as axial ligand are reduced at about −0.565 V, while for the PPh3 complex there is an anodic shift in the reduction wave and it is reduced at −0.169 V. This trend was also observed for the oxidation Co(II) → Co(III) + e− (Table 3; Fig. 8).

Fig. 8
figure 8

Cyclic voltammogram of (a) [Co(MeSalophen)(PR3)(CH3OH)]ClO4, a (R = Ph) and (b, dash line) (R = Bu) in DMF at room temperature. Scan rate: 100 mv/s

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In general, the reduction potentials are influenced by the axial ligand substitutions more than equatorial ligands. This investigation is consistent with electron transfer to dz 2 by the formation of Co(II). There is an anodic shift of peak potentials (E pa) for the complexes with the same axial ligand and functional groups such as NO2 and Br on the Schiff base; thus, the MeSalophen-substituted complexes are reduced easier (Table 3). This phenomenon is caused by the electron-withdrawing character of the substituents, which decreases the electron density on the metal center [40].

Conclusions

Taking into consideration the spectroscopy, structure and electrochemical properties of the [CoL(PR3)CH3OH]ClO4 complexes, the following conclusions have been made.

The spectroscopy and X-ray crystallography results confirmed the existence of Schiff base, phosphine, coordinated methanol and counterions. The X-ray crystallography results pointed out starkly that the synthesized complexes were hexa-coordinated in solid state and cobalt atom shown a distorted octahedral geometry. The cyclic voltammetry of the synthesized complexes shows that the anodic peak potentials E pa of different substituents increase via increasing the electron-withdrawing property of the Schiff base.

Supporting Information Available

Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication CCDC-1475925 for [Co(5-NO2MeSalophen)(PBu3)CH3OH]ClO4, and CCDC-1475926 for [Co(5-BrMeSalophen)(PPh3)(CH3OH)]ClO4. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [e-mail: deposit@ccdc.cam.ac.uk].